FORMING
ROLL-FORMING
STEEL STRIP
TECHNICAL BULLETIN
FTB-5
Rev 3, November 2010
This issue supersedes all previous issues
1 THE ROLL-FORMING PROCESS
Roll-forming is a process by which strip is
passed through a number of pairs of mated
profile rolls. The strip is either unwound from
a coil or sheet fed into the machine. Each roll
pair is designed to progressively develop a
profile by predetermined increments (Figure
1).
Roll-forming machines can produce sections of
almost any shape and dimension, consistently
and within close tolerances.
Roll-forming equipment, tooling and power
usage are relatively expensive. In assessing
the viability of roll-forming a profile which
could be produced by alternative methods, the
following points must be considered:
Roll-forming is a high productivity process
capable of achieving close tolerance control,
provide economy in labour and materials and
has the ability to produce long continuous
lengths and profiles of a complexity beyond
the capacity of the press bending process.
• Relative costs of equipment, tooling and
labour
In roll-forming, rolls gradually and
progressively change the shape of a section,
but the thickness of the metal generally
remains constant and equal to that of the flat
strip being fed into the machine.
The variety of products roll-formed from steel
strip and their fields of applications in all types
of industry are almost infinite. Some idea of
the variety of section profiles which can be
roll-formed is given in Figure 2.
Thus it is a bending not a stretching process,
nor a rolling process in the usual sense in
which a material is squeezed between rolls to
reduce thickness. The gap between forming
roll surfaces is ideally equal to (in practice
always slightly greater than) the overall
thickness of the material so that no reduction
can take place. In fact, any presence of cold
reduction in a roll-forming process is highly
undesirable. It is often caused by some
malfunctioning such as material pick-up by
rolls, incorrect roll setting with regard to height
or alignment, bad design or bad manufacture.
When purchasing or setting up a roll forming
line, it is very important to have set up charts.
These charts should tell the operator:
Figure 1 – Typical roll-forming stages
• Relative production rates of satisfactory
product
• Volume requirement.
• Where the critical alignment points are for
string lining
• Roll gaps for each stage and where to
measure these gaps
• Alignment procedure for idler rolls
• Alignment and setting procedure for shears
and other equipment used on the line.
Figure 2 – Typical examples of sections
produced by roll-forming
2.1.3 PRE-PUNCHING / NOTCHING
Some profiles, such as purlins have holes prepunched (punched while the strip is flat, before
it enters the roll-former) for the attachment
of bridging and for securing to the main roof
structure.
Other sections may require cut outs for
assembly or for ease of cutting after forming
and these cut outs are often punched into the
material before roll-forming. Suitable control
equipment is required to enable the prepunching units to operate in line.
2.1.4 OTHER
2 EQUIPMENT
The forming unit is normally a part of a
complete roll-forming line which consists of
an uncoiler, a cut-off device and a run-out or
stacking table. Some lines incorporate a roller
leveller, pre-punch unit and embossing unit
often located between the uncoiler and the
roll-former.
2.1 Uncoiler
If the roll-former is to be coil fed, then a
means of holding the coil and paying off
sufficient material is required. An uncoiler
usually performs this task.
There are a number of uncoiler types:
cantilever, cone, cradle, pallet and capstan.
The cantilever uncoiler is most often used in
high production roll forming because of the
ease in loading and unloading coils. Mandrel
expansion can be either powered or performed
manually.
Other processes can be incorporated into the
roll-forming line. These can include levellers
to condition the material before forming,
branding for recording the part number and
manufacturers details, embossing to form
a decorative pattern in the steel, welding
or other joining techniques and pressing or
folding operations after roll-forming.
2.2 Forming Stands
A roll-forming machine consists of several
stands, usually from 4 to 20 or more, each
supporting a pair of spindles (generally driven)
which carry forming rolls. Small shapes are
often produced with cantilevered (over-hung)
spindles, convenient for quick roll changes.
Medium and large sections require greater
forming forces, therefore the spindles need to
be supported at each end.
These two basic designs are shown
diagrammatically in Figure 3.
Figure 3 – Two basic roll-forming
machine designs
roll
space
Uncoilers can be either driven, in which case
a control is required to pay off the material at
the correct rate, or un-driven. The un-driven
uncoilers rely on the roll-former to provide
the power to unwind the coil. For un-driven
uncoilers a drag brake is required to stop
material being spilt onto the ground.
roll
space
2.1.1 COIL CAR
A coil car is often used in conjunction with
an uncoiler to load and unload the coils. The
coil car can increase throughput and also free
up lifting equipment that would otherwise be
waiting for the coil change.
2.1.2 UPENDER
Where coils are delivered bore vertical, and
the roll former requires bore horizontal for
operation, then upenders are required. A
properly designed upender can change from
bore vertical to bore horizontal quickly and
safely and eliminate damage to the coil outer
wraps.
(a) Cantilevered roll
spindles
(for small sections)
(b) Fully supported roll spindles
(for medium to large sections)
On the fully supported type, the inboard
supports are fixed to the machine bed in a
permanent fashion whereas the outboard stand
is made easily removable for roll changing.
An alternative to this is where the rolls and
spindles are removed from the stands for roll
changing. This requires the bridge caps to be
removed and the rolls and spindles removed
with a crane or other suitable lifting device.
Rolls are generally driven on the inboard side
either by a gearbox within the stand itself, by
a separate gearbox to each stand or by 1 or 2
gearboxes and chains driving the remaining
stages. To drive both top and bottom spindles,
either double output gearboxes or gears within
a stage can be used.
Single purpose units made for mass production
of a single profile can be of much simpler
construction without the need of provision
for a rapid roll change. The drive system can
be less elaborate and may be by chain and
sprockets or exposed spur gears.
Alternatively, if a frequent change from one
shape to another is required, roll-formers
may be provided with removable magazines
(also called cartridges). Separate magazines
carry several stands with fitted and preset
rolls, ready for production. Re-connection
of magazines may take some 20 minutes, as
against several hours for conventional roll
change and alignment.
2.3 Cut - Off Devices
When purchasing a new roll-forming line,
consideration needs to be made on whether
the line will be pre-cut or post-cut. Pre-cut
lines have a cut-off, usually a shear, located
before the roll-former and are effectively sheet
fed lines. Post-cut has the cut-off at the end of
the roll-former.
The main advantage of pre-cutting is the
simple cut-off shear that does not change
when the profile is changed. The main
disadvantages are the minimum sheet length
(3 times the stage spacing) and extra care is
required when designing the tooling to avoid
damaging the leading and trailing ends. This
usually results in additional stages being
required when compared to post-cut machines.
The advantages of post-cutting are the shorter
roll-former length, and the minimum cut-off
length is usually limited by guards and other
safety factors. The post-cut shear is however
more complex, expensive and is profile
dependent.
The cut-off devices used in post-cut operations
can be a saw, an abrasive disc, or a flying
punch and die cut-off unit. The latter type is
used for high capacity production since the
line does not have to be stopped to effect
a cut. Either an adjustable limit switch, or a
signal from an encoder actuates the press
automatically. The limit switch, which governs
the length of the product, is located on the
run-out table. The encoder, located within
the roll-former, sends a signal to a PLC or
computer which in turn tells the shear when
to cut.
The disadvantages of flying cut presses are the
relatively high cost, high noise and the need
for an individual pair of cutting blades for each
profile produced.
An improvement in computing and electronic
equipment and restrictions on noise limits has
resulted in the flying cut no longer being the
preferred method. Slow down to cut and stop
to cut units are becoming more popular as
they are quieter and more cost effective.
Saw cutting is suitable for very thick feed or
for closed profiles. A de-burring operation
is usually required after sawing, especially if
an abrasive disc is used. A slug punch shear
is suitable for generally open profiles of
approximately 100 mm depth and up to 3 mm
metal thickness. Swing or straight shearing
eliminates slug waste and slug disposal and is
used on open sections.
2.4 Straightening Equipment
Residual stresses from the forming process
are not usually symmetrical and provision of
equipment which can be adjusted to correct
torsional, vertical and horizontal misalignment
in the as-formed product is essential.
Straightening may be by an adjustable die plate
or by roller units in adjustable housings.
2.5 Forming Rolls
The main forming rolls can be either split or
solid (Figure 4). With split rolls the desired
profile at each stage is built up from a number
of segments whereas the solid is one roll
shaped to the required profile.
The split roll is the more widely used of the
two because:
a) roll manufacture is simpler
b) rolls can be designed for hardening and not
just to satisfy section profile
c) if a roll is damaged or badly worn only one
segment needs to be replaced, such as a
part susceptible to wear or damage
d) modifications can be easily and cheaply
effected
e) some rolls such as cone rolls or base rolls
can be used on a variety of sections.
Split rolls are built up from individual segments
which are tightened onto the spindle by a nut
at the outboard stand to produce the required
roll shape. For relatively small production
runs of simple shapes, rolls can be made
of unhardened, low carbon steel. Cast iron,
laminated plastic and even hardwood can be
used. The last two or three forming stages are
usually made of steel.
For high production and longer tool life, rolls
are made of high quality tool steel, usually
hardened to 60 HRC and ground after heat
treatment. Non-working roll segments and
spacers can be made of lower quality steel.
Recent trends in wide panel forming machines
has seen the use of high tensile steel which
can be readily surface hardened. The rolls are
usually hardened after commissioning trials.
The material must be guided laterally
throughout the roll-former. Initial location and
subsequent guiding are provided by the entry
guide, but additional guides may be required
between some forming stages. Further guiding
is generally carried out by the forming rolls
themselves, as the portion of section already
formed can provide an adequate lateral
location in subsequent stages. Guiding can
also be achieved by provision of locks on the
periphery of rolls as shown as Figure 6.
Before the rolls are placed in production, they
are normally assembled on dummy spindles
(or their own spindles) and checked for
accuracy of dimension and alignment. A wire
of a diameter equal to the thickness of sheet
to be formed can be passed between working
faces of assembled top and bottom rolls to
verify matching of surfaces.
Figure 6 – Guiding locks in rolls
Figure 4 – Roll-former roll types
solid roll
split rolls
(a) Lock recessed in bottom roll
3 ROLL DESIGN
In addition to the cross-sectional size and
shape of the product, factors which affect
roll design are steel strip thickness and width
tolerance, steel grade, strip edge condition,
required finish and accuracy, and the
anticipated production volume.
t
shoulder
height
= 0.8t
All these items must be finalised before detail
work is attempted.
(b) Locking by a separate roll
segment is a better solution
Basic design begins with an assessment of the
required number of forming stages (driven
rolls and idlers) and individual configurations.
In order to visualise the progress from flat
strip to final shape, intermediate shapes can
be drawn superimposed, whereby one obtains
flower diagrams such as shown in Figure 5.
Detailed consideration must be given to the
inward flow of material between the forming
stages. Too sudden a transition from one
shaped stage to the next can cause longitudinal
stretching of the material. This can introduce
residual stresses and give rise to a bow or twist
in the final product.
Figure 5 – Development flower diagrams
8 7
6
5
9
4
3
7
8
6
5
2
1
4
3
2
1
Rolls with locks are not popular among
manufacturers as they are more costly to make
and require strip feed slit to close tolerances.
Mill edge hot-rolled strip with a very wide
width tolerance for instance, is quite unsuitable
for guiding by roll locks. Where a feed with a
wide width tolerance must be accommodated,
the best solution is a self-centring entry guide
and rolls designed to provide adequate guiding
properties.
Ideally the peripheral speed of rolls closely
equals the speed of the material advancing
through the roll-former (roll-former speed).
However, as the roll diameters vary across
their width to suit the contour of each forming
stage, the peripheral speed of rolls is not
constant and thus cannot be made equal to
the roll-forming speed at each point. Rather,
an average or nominal pitch roll diameter
only can be matched with the material speed,
and the designer’s task is to determine such a
diameter for each pair of rolls. As a guide, the
pitch diameter can generally be taken as that
of the centroid of profile.
Rolls for deep profiles have a great discrepancy
between pitch diameter and maximum or
minimum working roll diameter. This results
in a substantial speed differential between
working surfaces, associated with heavy
Figure 7 – Forming path
H
F
th
le
pa
ng
i
rm
Fo
th
ng
ge
Unfortunately, there are few specific rules
which can be applied to the design of
roll profiles which will vary greatly with
complexity of section shape. Selection of the
number of stands, degree of bending per stand
and such factors will generally be based on
the experience of tool designers. For simple
shapes such as channels or angles, standard
roll-sets are available commercially, but for
jobbing work on more complex shapes,
in-house design of roll profiles is usually
required. It is possible to give a few general
guidelines to indicate some of the factors
which influence the design of roll-forming
rolls.
ed
This is the most important phase of rollforming since errors in tooling design can
result in high rejection losses, costly down-time
on the mill and little saleable product.
The continual improvement in computer
technology has seen the introduction of
design packages for roll forming. These
packages range from simple straight-line
approximations of material movement through
to more sophisticated analysis techniques.
These packages can be used to determine the
number of stages and the flower diagram.
ed
4 ROLL-FORMER DESIGN VERSUS
PRODUCT DESIGN
m
Any idler roll adjusts itself to the optimum
speed automatically and its mean diameter
is not critical. Sometimes rubbing can be
minimised by suitable orientation of the
product shape to reduce maximum roll
diameter, and consequently, differential speeds
between surfaces.
su
One remedy is good lubrication at the critical
spots (pump propelled jets of coolant/lubricant
are commonly used). Other methods include
felt wicks, and drip lubrication. Where possible
lubricants should be used sparingly. A number
of idler rolls on suitably angled spindles or
individual rolls mounted on bearings can also
be used to reduce scuffing.
For a simple channel of flange height H,
(Figure 7) the forming length may be
calculated by F=40 H. This assumes that if
the feed edge were to follow a straight line
in its flow through the forming stages, the
edge strain would be 0.06% which is less than
the elastic strain limit of steel. In practice the
edge path will be longer than the ideal. For
complex profiles the determination of the
forming length will be based on extending the
above geometrical approach, using imagination
and experience to devise a forming sequence
which will not induce plastic strain in
unformed elements of the profile. The use of
higher tensile material for roofing and walling
product, has resulted in modification of this
rule by some designers to 70 or 100 H.
As
rubbing which can give rise to product scuffing
and/or excessive roll wear.
H
4.1 Design of the Forming Stages
4.1.1 DESIRED SECTION
4.1.3 DETERMINATION OF THE
NUMBER OF FORMING STAGES
The section drawing should be complete in
specifying all pertinent information relative
to the dimensions and quality of the required
product. Tolerances must be known.
Assume a simple channel with a 50 mm flange
is to be roll-formed through forming stations at
400 mm intervals.
As well as product dimensions the designer
should consider longitudinal and cross bow,
camber and end flare and how these will
affect the end use of the profile. Specification
of tolerances for these attributes can assist the
designer in selecting an appropriate number of
stages and is beneficial when acceptance trials
are conducted.
4.1.2 DETERMINATION OF FORMING
LENGTH
The forming length is the distance between the
last point of unformed material and the point
at which the final form is completed.
First estimate the required forming length from
above:
F
=
40 x 50
=
2000 mm
Then from this, the number of station spacings
to give the number of stations:
Station spacings = 2000 ÷ 400
= 5 station spacings
= 6 stations
It is necessary to include an additional station
to fix the final profile, therefore a total of
seven stations is required.
4.1.4 DETERMINATION OF FORMING
SEQUENCE
a) bend angles selected to give a smooth
material flow; or
Station 1 will normally be used to align the flat
feed with the subsequent forming stations. All
forming stages are important but the final stages
should be designed to give a smooth exit to
the section rather than sharp profile changes
immediately before exit.
b) constant bend radii throughout the forming
stages.
Two alternatives of forming a 50 mm leg channel
illustrate the usefulness of a flow plan (Figure 8)
in showing the general flow pattern. For complex
flow plans, several significant points may be
drawn for a considered forming approach
and studied to determine whether the flow
pattern appears satisfactory or indicates where
modification would be desirable.
Figure 8 – Flow plans
aº
B
Improved A
A
B
1
0
0
2
40
18
3
60
36
4
75
54
5
85
72
6
90
90
7
90
90
A
A
4
3
5 6/7
2
a
1
B
5
4
6/7
3
2
1
a
Figure 8 shows that sequence A is preferred
because it gives a much better exit than sequence
B. Sequence A may be improved if a is reduced
slightly in stage 2.
In determining a forming sequence, the
properties of the sheet material need to be
considered.
For soft material such as G300, some designers
use:
a) a reasonably large forming angle in the
early stages.
b) radii larger than final form radii in early
forming stages.
These two factors are sometimes combined to
produce quite a large degree of lazy form at
the early stage. Usually, vertical offset is also
introduced to promote a smooth flow of feed to
optimise concentrated forming.
For softer materials some designers prefer to use:
For high tensile material it is generally desirable
to use:
a) bend angles selected to give a smooth
material flow, with reasonably small forming angle in the early stages; or
b) constant bend radii throughout the forming
stages.
Extra stages are often required when forming the
higher tensile material to overcome its additional
springback.
Good roll-forming will always require proper
consideration of the forming sequence, careful
alignment of uncoiler and all forming stations,
provision of side guides to promote smooth flow,
provision for correcting springback, adequate
straightening equipment and an efficient cut-off.
A little extra assistance to ensure good forming
is far less costly than losses in time, scrap and
goodwill when chances are taken by minimising
forming passes and auxiliary equipment.
It is very important to ensure the roll design
provides for the necessary profile length elements
(transverse) so the profile will be developed by
bending and not by stretching. Where stretching
does occur there will be a thickness reduction at
the bend radius rather than in the unbent areas.
There is also an increase in power required
for machines that stretch form. Normally this is
minor but if sufficient stretching occurs power
consumption can increase dramatically. This
thickness reduction causes a decrease in the
length of the bend (refer Figure 9) and the stress
imbalance across the profile can result in a variety
of shape problems, eg. oil canning. While this
condition can be recovered in ductile steels by
ensuring the rolls iron, and so extend the material
in the bend, the same recovery cannot be applied
to high strength products as discussed later. Care
should be taken when ironing the bends when
using organic and or metallic coated sheet steel.
The ironing can affect the life of these coatings.
Careful alignment of forming stages is essential
to minimise the possibility of excessive reforming
in the vicinity of bends. Correct alignment of the
lower spindle rolls may be achieved by relating
a selected point on the forming roll profile to
a common datum line through the mill. This
datum point may be on the faces of the inboard
rolls which abut accurately aligned roll spindle
shoulders. A stretched nylon fishing line can be
used to provide a datum line through the mill for
setting the positions of each roll.
Roll station drawings should include a specific
dimension from the selected profile point to the
datum point which is used in the setting up of
the stations.
Figure 9 – Effect of transverse stretching of
bends in a roll-formed profile
A decrease in full length of bend
resulting from a thickness reduction
can cause oil canning or looseness in
the flat pan of the profile
a) geometry of section - material thickness,
bend radius, bend angle and neutral axis
shift;
b) steel characteristics, i.e., yield strength and
thickness;
c) degree of conformity of section to contour
of rolls; and
d) type of bend and how formed. For
example, a bend or radius in the base of a
section can be stretched in if the strip edges
are held. This induces tension in the
material, and if excessive, causes permanent
set.
Stretching causes thickness reduction which, in turn,
causes a decrease in bend length
Top spindle rolls should be checked for proper
mating with the aligned lower roll profile. A thin,
flat strip of feed material or feeler gauges can
be used as a gauge to ensure that feed will have
sufficient clearance to pass between the mating
roll profiles and not be subjected to ironing
which may locally stretch the material and cause
oil-canning, edge wave or rippling.
Careful appraisal of space requirements should
be made to ensure efficiency. Relating production
rate of the mill to take off and disposal of
product will ensure correct space and handling
allowances. A smooth, comfortable running
speed continuously maintained is less fatiguing
for operators and equipment than stop-start
operation and overall production rate will be
higher.
The section drawing should be the focus for
all production considerations. It should give
all necessary information for feed supply, mill
selection, and tool design. The application of
tolerances should be rational. Roll-formed profiles
can be held to very close tolerances which are
important when there is assembly with other
components.
There are many requirements, however, where
quite liberal tolerances may be acceptable. If
so, it is unnecessary and costly to specify closer
tolerances which may result in high scrap and
low throughput.
Where a roll-formed section is to be further
processed (such as holed, notched, welded) this
information should be indicated on the section
drawing to properly relate the product through
the production phases.
4.2 Allowance For Springback
The springback effect due to the elastic
recovery of the steel strip can be allowed for by
overbending. This is normally done in the later
stages by side guides or vertical spindle rolls. The
amount of springback is dependent on many
factors:
If the same radius is formed without the edges
being held, little tension is induced. Thus the
amount of springback in the latter case would
be greater and increased overbending will be
required to form the design profile. Where
transverse stretching is applied to the feed as the
bend is being formed the springback effect is
reduced and stretching of the feed will occur in
the areas being bent (refer Figure 9). This will
mean that the bends will be shortened and there
will be a tendency to develop oil canning or
looseness in adjacent flat pans. This tendency can
be a considerable problem when using thin high
strength feeds.
4.3 Feed Width
The width of strip feed for a roll-forming
operation can be determined by adding together
the length of the flat elements of a roll-formed
shape and a length calculated from standard
formulae or tables for bend allowances. This
information is readily available in standard texts
on roll-forming.
4.4 General Considerations
It is possible to roll-form almost any profile, but
some shapes are inherently less troublesome
to produce or necessitate less costly rolls than
others. Thus, designers of new structural profiles
in particular, should familiarise themselves with
the fundamentals of roll-forming techniques and
consider the formability as one of the design
parameters.
The following additional information is provided
as a roll-formed product design aid:
a) profiles should be preferably open with side
angles at less than 90° (trapezoidal shapes);
b) where possible, one edge of the profile
should be free, that is, dimensions and
bend angle in the vicinity thereof should
not be critical. This will allow for feed
width inconsistencies and provide a scope
for compensation of some other possible
irregularities which may become evident in
trial runs;
c) wide, unsupported flats along either edge
should be avoided. They cannot be easily controlled in the roll-forming process and are
not structurally sound anyway;
d) sometimes it is difficult to eliminate residual
stresses even with great care in roll design
and setting; for example, roll-forming of
roofing and walling sheets which have wide
flat areas (pans or trays) in their profiles. One
remedy is the introduction of one or two
longitudinal flutes (shallow ribs) in the pans,
which may not completely eliminate residual
stresses but are effective in suppressing
or hiding the oil-canning tendencies. The
shallow flutes often may be drawn rather than
formed; and
Figure 10b. - Cross section of roll formed bend
(0.42mm bmt) with cracking of the paint.
Figure 10c. Cross section of roll formed bend
(0.42mm bmt) with cracking of painted and
metallic coatings.
e) structural and many other practical
considerations call for bend radii (inside radii
of bends) to be as small as possible, consistent
with the steel thickness and ductility.
Manufacturer’s data sheets should however
be consulted before the design is finalised to
determine recommendations.
If a material is bent to a radii greater than
that specified in BlueScope Steel’s Product
Data Sheets with no cross tension or base
metal thinning, then no cracking should be
observed on the bends of the metallic and
painted coatings. However, if the bend radii
are too tight, the paint and or metallic coating
may crack as shown in Figures 10a, b and c.
Paint cracking on light colours can be readily
seen with the naked eye or using a 10x
magnification eye-piece as shown in Figure
10a. In general, on similar bend radii, lighter
colours tend to exhibit more paint cracking
than darker colours. Also, strip temperature
has an effect on the potential for paint
cracking ie. more severe cracking will occur
on colder strip. NOTE – T-Bend adhesion and
cracking details quoted in relevant Product
Data Sheets, are based on tests performed at a
temperature as specified in AS/NZS 2728:2007
i.e. 23 +/- 5 deg C.
Figure 10a. - Magnified view (10x) of a roll
formed bend showing areas of cracking of the
paint.
The various stands of a roll-forming machine
need to be positioned to ensure that the flow
of material into the final shape is carried out
as smoothly as possible. In particular, the
following should be avoided:
a) excessive forming on any particular stand
which results in unnecessary stretching
of the base material often called “bend
thinning”. Figure 11 shows a cross section
of a bend with excessive bend thinning,
while Figure 12 shows a bend with minimal
stretching.
Figure 11 - Cross section of roll formed bend
(0.60mm bmt) with excessive bend thinning.
Figure 12 - Cross section of roll formed bend.
and other equipment used on the line.
5 ROLL-FORMING ORGANIC FINISHED
STEEL STRIP
b) inaccurate positioning of the stands in
relation to the flow of material, resulting in
re-rolling of the shape. This occurs where a
particular point of forming varies from stand
to stand, resulting in material being flexed
backwards and forwards, weakening the
bond of the coating as well as deforming
the base metal as shown in Figure 13. This
nominally 3 mm radius bend is composed
of a13
number
tighterofbends.
Figure
- Crossof
section
roll formed bend.
Organic-finished pre-painted strip is well suited
for roll-forming because of the lubricity of
the various coating types. However, common
sense precautions are necessary to ensure a
satisfactory performance of the coating. As is
the case when roll-forming any suitable type of
steel strip, the more forming stages, the greater
the ease of forming. Unnecessary deformation
of the steel base can destroy the adhesion of
the coating and therefore suitable roll-former
design is necessary. Roll-former settings are
mostly based on experience, with adjustments
being dictated by observation and the state
of the emerging formed product. As a general
guide, settings for organic-finished steel strip
should be calculated on the thickness of the
metal substrate only
(the steel plus any metallic coating present)
with the following considerations:
a) For COLORBOND® pre-painted steel strip,
disregard the thickness of paint coating.
If a coating of strippable polyethylene
film is present (CORSTRIP®), add 80% of
polyethylene film thickness.
b) COLORBOND® steel products should be
roll-formed dry. If a lubricant is considered
necessary for the painted surface of
COLORBOND® steel, SHELLSOL T (or
material with equivalent specification)
should be used sparingly.
c) Where COLORBOND® steel products carry a strippable polyethylene film no additional
lubrication is required as the polyethylene
film acts as a solid lubricant.
Severe bend thinning will significantly reduce
the strength of the base metal and may result
in splitting and/or other defects of the material.
d) Roll-formers should be cleaned free from all
traces of lubricants and metal debris which
may be present from any proceeding runs.
Any roll forming tooling needs to be regularly
maintained. Tooling should be checked
for wear and or damage on a regular basis.
Careful alignment of forming stages is essential
to minimise the possibility of excessive
reforming in the vicinity of bends.
This is particularly important for roll
forming COLORBOND® steel products after
galvanised material. Zinc particles from the
galvanised material can be transferred to the
COLORBOND® steel product by the roll
former tooling and result in unsightly dark
markings on the painted surface.
When purchasing or setting up a roll forming
line it is very important to have set up charts.
These charts or drawings should tell the
operator:
6 ROLL-FORMING HIGH STRENGTH
STEEL STRIP
a) The datum point which is used in the
setting up of the tooling.
b) Roll gaps for each stage and where to
measure these gaps
c) Alignment procedure for idler rolls
d) Alignment and setting procedure for shears
While the reduced ductility and higher yield
strength of the various uncoated and coated
high strength products ensures these cannot
be always successfully formed by conventional
roll-forming processes, experience gained in
recent years has provided the understanding of
both product characteristics and roll-forming
techniques to allow the production for a large
3
2
3
2
1
1
(a) Conventional forming of ductile steels
range of cladding and rainwater goods profiles
from high strength feed. The advantages of
these products are not only higher strength
and hence, reduced thickness, but decreased
damage during transportation, installation and
service.
These products are mainly ZINCALUME®
G550, ZINC HI-TEN® G550, ZINC HI-TEN®
G500 and ZINC HI-TEN® G450.
6.1 Comparisons With Conventional Forming
While high ductility, low strength products are
most readily roll-formed to structural sections
this ductility is associated with a low yield
strength which has an adverse affect on the
load bearing performance of the sections. The
increase in strength obtained by using cold
rolled stress relieved strip feed such as ZINC
HI-TEN® G500 and G450 steel is the most
economical and practical way to produce a
high strength section, and the roll-forming
industry is very able to produce the channel
and “Z” shapes required by this market.
Naturally, these high strength products cannot
be formed adequately using rolls designed to
form the ductile products and the following
comments provide an indication of the changes
to roll design concepts which are required to
produce these shapes.
One of the first lessons learned in roll-forming
high strength steels is that bends cannot be
readily reformed. The normal method for
ductile steels, Figure 14a, is to progressively
develop a bend from the adjacent unformed
flat, or to gradually reduce the radius. Up
to five stages may be used for a 90° bend.
Such gradual bend development allows the
surrounding material to travel through the
roll-former in a slow sweep, thus minimising
stretching near the toes of the channel and
residual stresses in the finished product.
When forming a channel of high strength
steel, a similar method can be used. The use
of variable radii forming should however be
avoided as the extra springback of the G550
material can cause problems. The springback
of the G550 material can mean that the initial
stages are operating in the elastic region and
the
latter
are required
far more
Figure
14 stages
– Forming
methodstofordoductile
and
work than the
designers
intended.
high strength steels
4
3
2
5
3
4 5
2
1
1
(a) Conventional forming of ductile steels
First stage
First stage
135º
Second stage
88º
Third stage
90º
Fourth stage
(b) Example of forming of high strength steels
In any set of tooling, care is required in tooling
alignment to ensure that re-rolling does not
occur as splitting may result.
Figure 14b shows an alternative method for
roll-forming high strength steels. In the first
stage, the material is deformed elastically,
taking care that the curvatures do not
introduce a permanent set. High yield strength
steels may, in fact, allow a fair transverse
curvature of the strip without permanent
deformation. The second stage produces a 45°
bend, with the edge sweep being minimal.
The third stage completes the bend with an
allowance for springback (92°-93°). The final
stage completes the channel shape. This
method requires four stages and probably also
one or two intermediate guides, but actual
bends are completed in two stages.
6.1.1 ROOFING AND CLADDING
SECTIONS
Stress relieved, high strength products such as
ZINCALUME® G550 steel and ZINC HI-TEN®
G550 steel have been used successfully for
many years to produce a corrugated profile.
Then the smaller radiused, prismatic profiled
shapes such as that used for TRIMDEK® and
SPANDEK® were converted to these products,
to be followed by flat-panned roofing shapes
such as KLIP-LOK® decking. Later there were
movements into various accessory shapes
including quad gutter.
The significant difference in forming
performance between these high strength
steels and the now replaced, soft products,
ZINCALUME® G300 steel and ZINCFORM®
G300 steel, is essentially springback. Further,
albeit smaller, increases of springback result
from the significant thickness reductions
achieved by the substitution of these high
strength products.
To compensate for the differences following
this product change, the basic concepts of
roll design need to be reconsidered. While
springback problems can usually be overcome
on the tightly radiused, prismatic shapes by
overbending techniques (and while these
correction methods are not critical on most
profiles) the forming principles used become
very important where there are flat pans
adjacent to the ribs, eg, in the KLIP-LOK®
shape.
6.1.2 PREFORMED BUBBLE TECHNIQUE
Roof and wall claddings, made of very
thin steel (0.42 mm base or less) with yield
strengths well above 550 MPa, allow for even
greater transverse deformations in the forming
process. One particular profile with prismatic
ribs has its strip preformed into bubbles before
the actual forming of bends begins (Figure
15). A suitable shape and size of the bubble
can be determined only by experience and/
or by a trial and error method. Fortunately, the
roll segments in pre-forming stages need not
be made of hard tool steel; thus they can be
readily re-machined or replaced during the trial
runs.
Figure 15 – Forming by means of a bubble
140º
140º
140º
6.1.3 OIL-CANNING
Oil-canning is the looseness which can
develop for a variety of reasons in the flat pan
of a roll-formed panel where it appears as
recurring waves of excess material along the
panel. This looseness can result from a number
of causes and these at times can be difficult to
isolate and identify.
One of these causes originates from incorrect
roll design or intermesh setting so that the
profile shape entering a later stage of the
former contains an excessive width of feed
or profile segment. As a result this excessive
width has to be crowded or crammed into a
smaller segment length and one possible result
of this action is to generate oil-canning.
A more common cause of oil-canning occurs
when forming flat pan profiles from thin high
strength products and excessively stretching
the feed over roll radii. Such stretching
is required to either overcome the high
springback of these products or because an
incorrect profile shape has been presented at
a later stage and must be stretched to produce
the specified shape.
When a bend is formed, in say 0.42 mm G550
feed, the bent area both thins and shortens.
Thus, in a formed panel, the bend will be
slightly shorter in length than the adjacent
pan. Should the transverse stress be increased
in the profile as the bends are being formed
and reformed, the degree of thinning and
hence the shortening, increases. Thus where
it is important to prevent the oil-canning of
adjacent flat pans, bend shortening should be
restricted as far as possible. This may mean
attention to the design concepts and operating
procedures. Some of these areas are as follows:
a) gathering of the strip around ribs by preshaping into a much looser shape as shown
in Figure 15.
b) provide sufficient width of feed in the initial
shaping stages of the forming operation
so that the required rib profile can be
developed without excessive stretching
within these areas. This will concern both
roll design and operating techniques.
c) form bends in as few passes as possible
and prevent re-rolling as will occur with roll
misalignment.
d) use an intermediate stage for guiding and
preforming the profiles and if necessary increase distances between stands so that
the feed can flow without restraint into the
component shapes.
Tension levelling of G550 feed can increase the
susceptibility of the feed to shorten at bends.
While this processing improves strip flatness
and hence the rollformed product for many
profile shapes, there may be a need to use a
product with modified levelling for those flat
pan profiles where freedom from oil canning
is required. Residual stresses in the feed after
modified levelling result in lower tendency to
oil canning. Where there is such a concern,
contact should be made with a local BlueScope
Steel Limited office.
It is important when selecting product
thickness, to recognise that the sheet or flat
pan stiffness or ability to resist oil-canning,
is directly related to the cube of the sheet
thickness, ie,
Stiffness = kT3
Thus a reduction of feed thickness from 0.48
mm to 0.42 mm reduces stiffness by 33% of the
original stiffness.
High strength products have been used for
applications such as rainwater gutters and a
variety of rainwater accessory shapes. When
these profiles have sharp bend radii and are
adequately stiffened there should be few
problems such as oil-canning provided there
are no basic roll design problems.
However, when large bend radii are required,
as with the quad gutter profile, there are
very likely to be difficulties in obtaining the
necessary shape without having to increase the
transverse tension in the profile while setting
the quadrant shape. The effect of increasing
this transverse tension is to reduce the
apparent springback of the feed so that less
over-bending is required. However the effect
of increasing the transverse tension while
bending, is to increase the bend shortening
which occurs and it is this shortening which
transfers the compressive stress into the
adjacent critical flat pan of the gutter. Thus
to minimise oil-canning in the quad gutter,
forming techniques will be required to set the
quadrant shape without causing undue bend
shortening.
Other factors that can influence oil-canning are
roll eccentricity due to bent spindles or poorly
machined rolls, stage to stage misalignment
and re-rolling.
6.1.4 RIPPLING OF EDGES
Rippling of edges can be influenced by a large
number of parameters within the roll former
and in the material.
Geometric considerations such as;
a) Location of holes in section. Holes close to
an edge can reduce the material’s ability to
resist stretching.
b) Unsymmetrical profiles where bend angles are the same but one edge is longer than
the other.
Machine conditions include:
a) Uneven forming and pressures, especially
on the edge of the profile resulting in stretching of the edge.
b) Uneven pass line height for tooling causing
the edge to travel further than the designer
intended.
c) Incorrectly set straightening device which
over stretches one or both edges (can be linked with trying to remove longitudinal
bow or twist).
d) Pre and post roll forming equipment not
aligned.
e) Too rapid a forming sequence can cause
stretching of the edges of the material.
f) Roll diameter too small. The larger the
diameter the greater the forming distance.
Material properties that can influence edge
wave are:
a) If the coil has re-crystallised (lower tensile
strength) and or thinner edges then this
material can be more susceptible to
stretching and hence edge wave.
b) Incorrect material selection. If the roll
former has been designed for high tensile material and low tensile material of the
same thickness is roll formed, then edge
wave may occur.
Note also that edge wave can be induced
through poor slitting practice.
c) Unstiffened flanges are more likely to suffer
edge wave. The provision of a stiffener
close to the edge can hide any edge wave.
The information and advice contained in this Bulletin is of a general nature only, and has not been prepared with
your specific needs in mind. You should always obtain specialist advice to ensure that the materials, approach and
techniques referred to in this Bulletin meet your specific requirements.
BlueScope Steel Limited makes no warranty as to the accuracy, completeness or reliability of any estimates, opinions
or other information contained in this Bulletin, and to the maximum extent permitted by law, BlueScope Steel Limited
disclaims all liability and responsibility for any loss or damage, direct or indirect, which may be suffered by any
person acting in reliance on anything contained in or omitted from this document.
COLORBOND®, CORSTRIP®, ZINCALUME®, ZINC HI-TEN® , ZINCFORM®and BlueScope are registered trade marks
of BlueScope Steel Limited. KLIP-LOK®, SPANDEK® and TRIMDEK® are registered trade marks of BlueScope Lysaght.
Please ensure you have the current Technical Bulletin as displayed at www.bluescopesteel.com.au
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